In semiconductor processing, many operations may be performed on a single workpiece or semiconductor wafer. In general, each processing operation on a workpiece is typically performed in a particular order, wherein each operation waits until completion of a preceding operation, thus affecting the time at which the workpiece will become available for a subsequent processing step. Tool productivity or throughput for relatively short processes performed under vacuum, such as ion implantation, can be severely limited if the process flow leading to the processing location is interrupted by sequential events associated with such processing. For example, operations such as an exchange of workpieces between transport carriers or storage cassettes and the processing system, a transfer of the workpiece from an atmospheric environment into an evacuated environment of an implantation chamber of the processing system, and an orientation of the workpiece (e.g., notch alignment) within the evacuated environment, can have a significant impact on tool productivity.
Processing of a workpiece, such as ion implantation, for example, is typically performed at a reduced pressure within an implantation chamber, wherein ions are generally accelerated along a beam line, and wherein the ions enter the evacuated implantation chamber and strike the workpiece in a predetermined manner. Several operations are typically performed leading up to the implantation in order to introduce the workpiece into the implantation chamber, as well as to properly position and orient the workpiece with respect to the ion beam within the ion implantation chamber. For example, the workpiece is transferred via a robot from an atmospheric cassette or storage device into a load lock chamber, wherein the load lock chamber is subsequently evacuated in order to bring the workpiece into the processing environment of the ion implanter. The cassette or storage device, for example, may be delivered to the ion implanter via a conveyor system or other type of delivery.
Front opening unified pods (FOUPs), for example, have become a popular mechanism for moving silicon workpieces or wafers from one workstation to another in an integrated circuit (IC) fabrication facility. Different versions of these FOUPs are commercially available from different manufacturers, including Asyst Technologies and Brooks Automation. A FOUP containing a number of stacked wafers, for example, is delivered from one tool to a next subsequent tool by an automated delivery device such as an overhead transport. The overhead transport deposits the pod to a location within the reach of a robot so that a robotic arm can extract one or more silicon wafers from the pod for treatment.
U.S. Pat. No. 5,486,080 to Sieradzki, for example, details a system for transferring wafers for vacuum processing. The system employs two wafer transport robots for moving wafers from two load locks past a processing station. Additional patents relating to serial end stations are U.S. Pat. Nos. 6,350,097, 6,555,825, and 5,003,183. Further, commonly-owned U.S. Pat. No. 7,010,388 to Mitchell et al. details a wafer handling system for handling one or two wafers at a time.
It is desirable for the workpiece handling system to have very high throughputs in order to reduce the tool's cost of ownership. This is especially true in an ion implantation process when a duration of the implantation is very short compared to the time needed to transfer a new workpiece from the FOUP to the process chamber and back to the FOUP. The actual ion implantation into a workpiece for a low dose implant, for example, has a short duration, wherein implant times can be less than 5 seconds. Further, as part of pre-processing routines utilized in ion implantation, each workpiece must be oriented properly relative to the ion beam. A mechanism known as an aligner is used for such an alignment step, where each workpiece is aligned serially, thus potentially decreasing throughput.
Therefore, a need exists for a system and method for facilitating high throughput by allowing simultaneous placement of two workpieces by an atmospheric wafer handling robot at the aligner station for subsequent serial alignment by the aligner mechanism. Since two wafers can be dropped off simultaneously, the atmospheric wafer handling robot can proceed onto other tasks which allow the wafer robot to handle more wafers per hour.